The invention relates to a coding and decoding process and device compatible with the MPEG2 standard for displaying a zoom of a transmitted image. Performing a zoom of an image consists in enlarging or reducing a part of this image. It therefore involves a modification of the horizontal and vertical dimensions of the image part selected for display in zoom mode.
Nowadays, applications based on interactivity necessitate the ability to view images in reduced or magnified form. For example, electronic program guides utilize images of reduced sizes so as to allow display on the screen of ancillary information in OSD form (standing for On Screen Display). Magnification may also be necessary in the case where one wishes to focus the display on a part of the source image.
The process proposed can be used by any type of apparatus utilizing a stream of data coded according to the MPEG2 standard, for example a decoder, a DVD reader/recorder, an Internet box, etc. The transmission of the data may be transmission by satellite, by cable, terrestrial transmission, etc.
MPEG2 compression is not an object-based processing and the decoded image displayed is generally a copy of the coded source image. To achieve a specific display in zoom mode, it is therefore necessary to code the source image such as it must be displayed, that is to say after having performed the zoom on this source image. Thus, the source image, originating for example from a studio providing a video sequence, is processed before the coding and transmission of the video data. The zoom operations are performed on this source image and it is the new “zoomed” image which is coded like the other images of the video sequence. The drawback of such a process stems from the lack of flexibility which it imposes. The zooms to be carried out must be decided on at the time of coding. These zooms can then no longer be modified on receipt of the data, at the decoder level, for example as a function of data or parameters selected by the viewer.
The aim of the invention is to alleviate the aforesaid drawbacks.